The methyl ester route for soap making involves first the preparation of fatty acid methyl esters from fats and oils (ester exchange) and subsequent saponification of the methyl esters to give soap with concomitant recovery of ensuing methanol, all according to the following. ##STR1##
Displacement of the glycerol in a fat by a low-molecular-weight alcohol, such as methyl or ethyl alcohol, is described in Bradshaw and Meuly U.S. Pat. No. 2,271,619. The process is said to produce methyl or ethyl esters directly from the fat, without intervening hydrolysis, and is said to take place at low temperatures. Although the reaction can be carried out in open tanks constructed of ordinary carbon steel, it is preferred to use sealed vessels. The fat must be clean, dry, and substantially neutral. It is heated to about 80.degree. C. (176.degree. F.), and to it is added commercial anhydrous (99.7%) methyl alcohol in which is dissolved 0.1-0.5% sodium or potassium hydroxide. The quantity of alcohol recommended is about 1.6 times that theoretically required for the reaction, although it is stated the alcohol may be reduced to as little as 1.2 times theoretical, if the operation is carried out in three steps. Alcohol amounting to more than 1.75 times the theoretical quantity does not materially accelerate the reaction and is said to interfere with subsequent gravity separation of the glycerol.
The Bradshaw patent contemplates use of the methyl esters to make anhydrous soap by a continuous process. It is stated the esters are saponified by caustic soda or caustic potash at a low temperature, and the methyl alcohol can be recovered for reuse. The methyl and ethyl esters of fatty acids are fluid, relatively stable, noncorrosive, and low-boiling derivatives, and in certain operations are preferred to free fatty acids. Methyl esters are preferred over the ethyl esters for reasons of lower cost of manufacture and better pyrolytic stability during processes such as fractional distillation.
It has also been reported that the alkali-catalyzed alcoholysis method is completely successful only if the fat is almost neutral and the reaction mixture is substantially anhydrous. Failure to comply with either of these conditions causes soap formation, which leads to a loss of alkalinity and also the building up of a gel structure that prevents or retards separation and settling of the glycerol.
The saponification of fatty methyl esters with alkalis to produce soap is well-known. Equipment for this purpose is available, for example, from Lion Corporation in Japan and Ballestra in Italy. In the known processes for manufacture of soap from fatty methyl esters, the methyl esters are first reacted with an alkali which results in the production of a soap mass containing both water and methanol. In the next step, excess water and methanol are removed. Several procedures are available to accomplish this step. For example, methanol can be removed by placing the soap mass as a thin film on a rotary drum. The soap mass is thus converted to soap flakes which can then be dried further by passing the flakes through an oven.
However, the following disadvantages are noted for the saponification of methyl esters to produce soap by such prior art methods:
1. The reaction of methyl esters with alkalis cannot be accelerated since all reactions are done at ambient pressures. PA1 2. The concentration of soap in the soap mass is usually limited to the 60-70% range in order to have proper fluidity of soap mass to flow. PA1 3. The drying of soap mass to remove methanol and excess water is highly limited and often not easily controllable. PA1 4. The recovery of methanol for recycling into the system is rather complicated involving multistep processing.